ImDrawListSplitter: fixed an issue merging channels if the last submitted draw command used a different texture. (#2506)

imgui_draw.cpp

@@ -1301,6 +1301,7 @@ void ImDrawListSplitter::Merge(ImDrawList* draw_list)
     }
     draw_list->_IdxWritePtr = idx_write;
     draw_list->UpdateClipRect(); // We call this instead of AddDrawCmd(), so that empty channels won't produce an extra draw call.
+    draw_list->UpdateTextureID();
     _Count = 1;
 }